Detectors may be fabricated in many ways, and may serve many purposes. For all detectors, sensitivity and signal-to-noise ratios are important to successful operation. Radiation photodetectors can be constructed with photoconductor sensors. When attempting to detect x-rays, photodetectors are preferably highly sensitive to x-rays and relatively insensitive to other electromagnetic radiation. The photoconductors can either be intrinsic semiconductor materials that have high resistivity unless illuminated by X-ray photons, or diode structures that have small currents due to the blocking effect of the diode junction unless illuminated with X-rays.
FIG. 1A illustrates one type of conventional photodetector 50 that includes a semiconductor material with a pair of contact electrodes on either side of the semiconductor material. The semiconductor material acts as a direct conversion layer to convert incident radiation to electric currents. A voltage source connected to the electrodes applies a positive bias voltage across the semiconductor material, and current is observed as an indication of the magnitude of incident radiation. When no radiation is present, the resistance of the semiconductor material is high for most photoconductors, and only a small dark current can be measured. When radiation is made incident through the top contact electrode upon the semiconductor material, electron-hole pairs form and drift apart under the influence of a voltage across that region. Electrons are drawn toward the more positively (+) biased contact electrode and holes are drawn toward the more negatively biased (e.g., quasi-grounded) contact electrode. Formation of electron-hole pairs occurs due to interaction between the incident radiation and the semiconductor material. If the x-rays have energy greater than the band gap energy of the semiconductor material, then electron-hole pairs are generated in the semiconductor as each photon is absorbed in the material. If a voltage is being continuously applied across the semiconductor material, the electrons and holes will tend to separate, thereby creating a current flowing through the photodetector. The magnitude of the current produced in the photodetector is related to the magnitude of the incident radiation received. After removal of the incident radiation, the charge carriers (electrons and holes) remain for a finite period of time until they either reach the collection electrodes or can be recombined. The term “charge carriers” is often used to refer to either the electrons, or holes, or both.
Some semiconductor materials used for x-ray detectors are selenium, mercuric iodide and lead iodide. The two iodide compounds have a higher mobility product, require a much lower charge collection voltage than selenium, and have additional advantages such as greater temperature stability. However, each of mercuric iodide and lead iodide has physical parameters that affect their performance and ease of use in single layer x-ray detectors.
In mercuric iodide, the carrier mobility is measured to be higher than lead iodide and the lag time is found to be lower. The lower carrier mobility means that it is difficult to use a thick layer of lead iodide, which is more efficient in absorbing a greater fraction of incident x-ray photons, especially at higher photon energies that increase detector sensitivity. However, mercuric iodide is more chemically reactive toward typical contact materials (e.g., aluminum) than is lead iodide and considerable problems have been experienced with contact corrosion in flat panel detectors coated with mercuric iodide.
As mentioned above, photoconductors may also have diode structures based on either a p-i-n or p-n configuration. FIG. 1B illustrates a conventional p-i-n diode. Such a photodiode 100 is termed a “p-i-n” diode for the configuration of semiconductor material in the diode. Photodiode 100 is composed of a p-doped semiconductor (p-type) material layer 110 and an n-doped semiconductor (n-type) material layer 130. Light is made incident on the depletion region between the p-type and the n-type material layers, creating electron-hole pairs and thus a current. To control the thickness of the depletion region, a layer 120 of intrinsic (i) material is inserted between the p-doped semiconductor material layer 110 and the n-doped semiconductor material layer 130. This structure may be used to detect an x-ray which is incident on either the p-doped semiconductor 110 or the n-doped semiconductor 130. Photodetectors based on a p-i-n structure also include contacts to apply bias to the material layers, as illustrated in FIG. 1C. Photodetector 150 includes a top contact conductor 181 connected to p-doped region 182 and a bottom contact conductor 185 connected to n-doped region 184. P-doped region 182, intrinsic layer 183 and n-doped region 184 are all semiconductor materials as described with respect to detector 100. The layers are formed on a substrate 186 that acts as a base for the detector 150.
As mentioned above, the p-i-n structure may be used to detect x-rays that are incident on either of the p-doped semiconductor material layer 182 or the n-doped semiconductor material layer 185. In operation of p-i-n photodiode 150, a reverse-bias voltage is applied across the photodiode and x-rays are made incident upon the intrinsic region 183. The electron-hole pairs then separate under the applied electric field and quickly migrate toward their respective poles. The electrons move toward the positive pole and the holes move toward the negative pole. Conventional photodiodes have narrow intrinsic regions 183. Due to the narrowness of the intrinsic region 183 and also due to the high mobility of the intrinsic material, there is little chance that the carriers will recombine before they arrive at the interface with the doped material. The electrons and holes then collect near the respective interface with the doped material. The change in resistivity results in a change in one or both of a voltage or current between top conductor 181 and second conductor 186, which may be measured in a surrounding system (not shown).
One problem with prior diode structure photoconductors is that dark (leakage) current limits the usefulness of the high x-ray sensitivity of photoconductor sensors. One solution to substantially reducing such dark current is by using p-n heterostructures of photoconductors. Diodes structures (p-n and p-i-n) may be composed of two or more dissimilar semiconductor materials, thereby forming a heterojunction. For example, one prior photodetector consists of a layer of cadmium telluride and a layer of cadmium sulfide forming a heterojunction. The cadmium telluride is deposited so that it is a p-type material (excess holes) and the cadmium sulfide is deposited so that it is an n-type material (excess electrons). An external voltage applied across the heterojunction of the two materials produces a reverse biased p-n junction that acts as a photodiode. As discussed above, radiation induced electron-hole pairs give rise to electrical currents that flow in proportion to the incident radiation. The p-n junction, when reversed biased, inhibits dark current from flowing across the junction.
The performance of a photoconductor may be judged by various criteria including sensitivity. Sensitivity refers to the current produced by a photoconductor with respect to the electromagnetic radiation intensity. A photoconductor with high sensitivity will produce more current for a given intensity of incident radiation than one with a low sensitivity. Sensitivity is affected by many factors including the mobility of the electrons in the material. Semiconductor materials with a higher mobility have a higher sensitivity, if other parameters are similar, because the electrons can move at a greater speed.
Detectors may be capable of sensing mega-voltage (MV) energy x-ray photons if enough of the x-ray radiation can be absorbed and converted into electron-hole pairs (free charge) by a photoconductive conversion layer or absorbed and converted into photoelectrons, in the underlying substrate layer, that then pass up into and are detected by the photoconductive conversion layer. Typically, a high density substrate is used to provide such absorption and photoelectron generation. One problem with many high density substrates is that they are limited in the number of pixel elements they can incorporate or are limited in the physical dimensions of their total detector area (in contrast to a pixilated array of pixels of electronics, such as thin film amorphous silicon transistors, capacitors, switches, and amplifiers on low density glass substrates which can accommodate many millions of individual pixels and hundreds to thousands of square inches of detector area). Moreover, fabrication costs of the high density substrate detectors is more expensive than for fabrication of low density substrates.